It's probably cheating a little to just grab an existing Javascript interpreter written in C++, compile that with emscripten and call it a Javascript interpreter in Javascript. When you compile C++ code, do you say that the program is in Assembly?
Agreed, it's partially cheating, although a lot of work went into getting it to compile to LLVM and patching it work with Emscripten. That's not where it stops though. The plan is to make the library work just like the SpiderMonkey JS API, which will require wrapper scripts in JS.